We have memberships that are "fixed" from Jan 1 through Dec 31. This works fine except for that we want that if anyone creates or renews a membership in November or December, that their membership will be valid for this year and next year.
So if I create/renew my Membership in September 2019, I will be a member from then until December 2019.
But if I create/renew my Membership in November 2019, I will be a member from then until December 2020. Here I will get Nov/Dec of this year "for free" and then be a member for all of next year.
Presently, if I create/renew a Membership in November 2019, it ends December 2019.